A recent study found activating Notch1 mutations in 55% of human T-ALL patient samples. These newly discovered mutations are distributed among two hotspots -- the heterodimerization (HD) and PEST domains. However, the relative importance and consequence of each mutation remains unknown and unexplored. My preliminary research suggests that a single activating mutation in the HD hotspot cannot induce T-ALL in mice unless a second mutation occurs in the PEST hotspot in cis. I will follow up on these observations by determining whether all HD mutations require concurrent PEST mutations and by identifying cooperating oncogenes during the multi-step pathogenesis of T-ALL. In order to understand the molecular mechanism by which Notch initiates and sustains oncogenesis, I will characterize the effect of Notch inhibition on the proliferation, survival, and differentiation of leukemic cell lines and use this information to develop a systematic strategy for testing putative target genes generated from microarray data sets. Together these studies should not only lead to an improved understanding of T-ALL pathogenesis but will also identify novel therapeutic strategies for treatment. This proposal describes a 5 year training program to develop an academic career in cancer biology. The experiments described in the proposal will provide valuable experience in the use and manipulation of murine genetic model systems for the study of cancer under the mentorship of Dr. Warren Pear. Dr. Pear is an associate professor at the Abramson Family Cancer Research Institute with extensive experience in the manipulation and analysis of animal models of leukemogenesis. Ultimately, the time spent in the development of this project will establish a line of inquiry in the field of leukemogenesis and will provide the skills necessary to establish myself as a successful physician scientist and independent researcher. Acute T cell lymphoblastic leukemia remains a deadly cancer in children and adults which cannot be cured in more than one out of every three to four patients. A recent breakthrough study has identified Notch1 mutations in the majority of patients with this cancer. My research seeks to uncover the significance of these mutations and the cancer-causing molecular events in the cell that are triggered by this mutation, which may provide new opportunities for therapy.